What is water hammer effect?
When the water flowing in a long pipe is suddenly brought to rest by closing the valve, the momentum of the flowing water will be destroyed and consequently, a wave of high pressure will be set up. The kinetic energy of flowing liquid will be converted into the internal pressure energy with the rising of pressure.
This wave of high pressure will be transmitted along the pipe with a velocity equal to the velocity to the velocity of sound and may create a noise called knocking. This phenomenon of a sudden rise in pressure in the pipe is known as a water hammer or water blow.
Cause of water hammer
Water hammer is caused by changes in velocity which are caused by:
- valve operation (i.e closer and opening of valves)
- Power failure
- starting or shut down of pumps (hydro turbines)
- Fluctuation in power demand in turbines
- Rupture of the line. etc.
- Mechanical failure of the control devices like the valve.
Effects of water hammer
- High-pressure fluctuations in pipelines
- Rupture of pipe or valve if beyond the safety limit
- Higher pressure requirements for the design of pipelines and penstocks etc.
The magnitude of pressure rise depends on
- Time is taken to close the valve
- Velocity of flow
- Length of pipe
- Elastic properties of the pipe materials as well as that of the flowing fluid.
Water hammer formula
Time required by pressure wave to travel from the valve to tank to valve.
Time is taken = Distance travelled from valve to tank and back/ Velocity of pressure wave
T = (L+L)/C = 2 L/C
Where L is the length of the pipe
C = Velocity of pressure wave
1) The closure of the valve is said to be gradual when T>2L/C
2) The closure of the valve is said to be sudden when T<2L/C
Where t = time required to close the valve, C- velocity of pressure wave
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